INTRODUCTORY 3 



cells, derived from more or less distinct sources, to form one 

 new cell. The parental cells which thus fuse are called 

 sexual cells, or gametes, which are detached from the 

 parental body. The result of their fusion is called a 



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A 



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FIG. i. 



A, Five young vegetable cells, each with its granular protoplasm surrounding 

 a large spherical nucleus. Each cell is delimited by a cell-wall which is thin while 

 young, but may become much thickened as the cell becomes mature. ( x 800.) 



B, Part of a transverse section of the stratified, epithelial lining of a cat's 

 oesophagus, b.m., basement membrane. Here each cell is not delimited by a cell- 

 wall, as is the case in plant-tissues. ( x 700. From Dahlgren and Kepner.) 



zygote, and it forms the starting point for the develop- 

 ment of a new individual. This individual may remain 

 unicellular, as in the case of the simplest living things. 

 Or, as in higher organisms, it may undergo cell-division ; 

 and this if continued may give rise to a large and complex 

 individual, such as a horse or a tree. But whatever the 

 size or form of the sexually-produced offspring may be, 

 the genesis of it is essentially the same, by fusion of two 



